1. Field of the Invention
The present invention relates to a seat belt retractor and more specifically to a seat belt retractor which can increase the durability and the seat belt take-up strength and decrease the cost thereof.
2. Description of the Prior Art
At present, the performance of automotive vehicles have been improved more and more, and further the driver must legally wear a seat belt on his body to ensure driver's safety. Therefore, there exists a strong demand for improvement of the seat belt. A seat belt retractor constitutes an essential portion of the seat belt, and generally provided with a drum rotatably supported by a frame, for winding a seat belt around the outer circumference thereof; an emergency lock mechanism disposed on one side of a drive shaft of the drum, for stopping seat belt let-out motion from the drum at an emergency; and a seat belt take-up mechanism disposed on the drive shaft of the drum, for always applying a return force to the seat belt via the drum.
In the above-mentioned construction, the emergency lock mechanism for stopping seat belt let-out motion at an emergency to ensure driver's safety has been improved widely, and a number of mechanisms have been proposed.
However, the seat belt take-up mechanism for always applying a return force to the seat belt has not been improved, in spite of the fact that this mechanism contributes to the feeling of the seat belt user, because this mechanism does not directly relate to the safety improvement.
Conventionally, a flat spiral spring G as shown in FIG. 1(A) has been used as the take-up force applying means of the seat belt take-up mechanism which exerts an influence upon the feeling of the seat belt user. This flat spiral spring G is manufactured by winding a thin strip-shaped spring material into a spiral shape. Therefore, since there inevitably exist edge portions e along the longitudinal direction thereof as shown in FIG. 1(B), there exists a problem in that internal stress will be concentrated to the edge portions e of the flat spiral spring G when the seat belt is being used and therefore the spring G is often broken, so that the durability of the spring G is not sufficiently long.
To overcome the above-mentioned problem, a method of chamfering the edge portions e has been proposed. However, this method increases the manufacturing cost thereof and further it is difficult to obtain a uniform quality. In addition there exists another problem in that the diameter of the flat spiral spring G inevitably increases with increasing take-up torque for the seat belt.